The availability of safe and effective birth control is of vital importance to most young women, and over half of U.S. women who use reversible contraception are oral contraceptive (OC) users. While OCs have the lowest theoretical failure rates of any reversible method recommended for women planning future childbearing (0.2% per year), actual OC failure rates are 10 to 15 times higher, resulting in 500,000 unintended pregnancies among U.S. OC users annually. This discrepancy has not resulted in much epidemiologic research into risk factors for OC failure, as most failures are assumed to be due to patient misuse (user failure). However, OC dose is not titrated to patient weight, and preliminary research indicates that body habitus may have a dramatic impact on the likelihood of OC failure. Consequently, the investigators propose a population-based case-control study among reproductive-age women enrolled in a health maintenance organization in western Washington State to test the hypothesis that obesity may increase risk of oral contraceptive failure, taking into account both user and method failure. The associations between OC failure and other factors potentially affecting OC metabolism and estrogen production (smoking, exercise, medications, diet) will be investigated as well. Potential cases will be all 18-39 year old enrollees of Group Health Cooperative of Puget Sound (GHC) who are OC users with a concurrent positive pregnancy test between September 1, 1997 and August 31, 2000, selected using computerized pharmacy and laboratory records with eligibility verified by medical record abstraction (n=600). Controls will be an equal number of non-pregnant OC users, randomly selected from computerized pharmacy files and frequency matched to cases on age, primary care clinic region, and OC use duration. Data will be obtained from cases and controls by in-person interview and dietary questionnaire, and linked with the GHC computerized pharmacy database. Subjects will be interviewed regarding patterns of regularity of OC use (e.g., skipped or irregularly timed doses) and body habitus details (height, weight, weight gain patterns), anthropometric measurements will be obtained, and other risk factors for OC failure such as chronic disease, medication use, and illness will be assessed.